This invention relates to an improved current probe.
U.S. Pat. No. 3,525,041 discloses a current probe measurement system comprising a ring-shaped core of magnetic material defining an aperture through which a conductor carrying a current to be measured extends. A multi-turn winding is wrapped around one leg of the core. The current to be measured produces a magnetic flux in the core, and the flux in the magnetic core links the winding. Thus, the current-carrying conductor, the core and the multi-turn winding function as a transformer, in which the current-carrying conductor is the primary and the multi-turn winding is the secondary. The flux in the magnetic core also links a thin film semiconductor Hall device, which has a first pair of opposite terminals connected between a bias current source and ground and a second pair of opposite terminals connected between ground and an input of an amplifier. The output of the amplifier is connected to one end of the winding, and the opposite end of the winding is connected to a voltage measurement instrument across a load or termination resistor.
A high frequency component of the current in the primary conductor results in a current being induced in the secondary winding in a direction such as to generate a magnetic field in the core that is opposed to the field created by the current in the primary conductor. A low frequency or DC component of the current in the primary conductor is less effective at inducing current in the secondary winding, but generates a potential difference between the second pair of terminals of the Hall device, and the amplifier provides a corresponding current to the winding. The direction of the current supplied by the amplifier is such that the magnetic field created in the core by the current flowing through the winding is opposite to the direction of the magnetic field created by the current in the primary conductor. Over a wide range of frequencies, the voltage developed across the load resistor is representative of the current in the primary conductor.
FIG. 1 illustrates a practical implementation of the current probe shown in U.S. Pat. No. 3,525,041, for example as applied to current probe measurement system based on the A6302 probe, AM503 plug-in amplifier and TM500 power supply manufactured by Tektronix, Inc. of Wilsonville, Oreg. As shown in FIG. 1, the Hall device 10 provides a differential input signal to a Hall pre-amplifier 14, whose output is applied through a resistor 18 to a power amplifier 22 that is provided with a feedback resistor 26. The output of the power amplifier 22 is connected through one conductor 28 of a cable 30 to one end of the secondary winding 34, and the opposite end of the winding 34 is connected through another conductor 38 of the cable 30 to the non-inverting input terminal of a differential scaling amplifier 50 having its inverting input terminal grounded. A termination resistor 52 is connected between the input terminals of the scaling amplifier 50. Accordingly, the scaling amplifier receives as input signal the voltage developed across the termination resistor, which is proportional to the current provided by the power amplifier. The output voltage of the amplifier 50 is proportional to the input voltage and is applied through a vertical deflection circuit 56 to a cathode-ray tube display device 56. A transformer shunting device 57 may be connected in parallel with the winding 34, and an output shunting device 59 may be connected in parallel with the series combination of the conductor 38 and termination resistor 52.
A generalization of the current probe measurement system shown in FIG. 1 may be represented by a current-responsive system of the form shown in FIG. 2. FIG. 2 shows a current-to-voltage converter 58 that senses the current flowing in the primary conductor 8 and provides a voltage signal that is related to the current Ip by the transfer function T volts/amp. The output signal of the current-to-voltage converter 58 is applied to the scaling amplifier 50 having a gain A.sub.s and providing a voltage output signal V.sub.o. The voltage output signal V.sub.o is applied to a voltage-sensitive output device 61 having a sensitivity S.sub.o output units/volt. Accordingly, the overall transfer function of the current-responsive system is T*A.sub.s *S.sub.o output units/amp, and the output value is given by: EQU OUT=I.sub.p *T*A.sub.s *S.sub.o
The voltage-sensitive output device might be, for example, a voltage measuring device such as an oscilloscope display mainframe, in which case the output value would be a number of display divisions on the oscilloscope screen, or it might be an analog-to-digital (A/D) converter, in which case the output value would be a number of least significant bits in the output of the A/D converter. However, the current-responsive system is not restricted to the output value being quantized.
It is generally desirable that the overall transfer function of the current-responsive system have a convenient value that is known exactly. In particular, in the case of the output device being an oscilloscope display mainframe it might be desirable that the overall transfer function be a convenient number of display divisions/amp. The value of S.sub.o is usually convenient and known exactly, and the value of A.sub.s can be given a convenient and exactly known value by using precision components in the scaling amplifier. However, the value of T might not be known exactly and might not be convenient.
Referring to FIG. 1, it can be shown that although the ideal transfer function T.sub.ideal relating the current in the primary conductor 8 to the voltage developed across the termination resistor 52 is equal to Rt/N volts/ampere and is thus independent of Hall gain of the Hall device 10, the actual transfer function T.sub.real is dependent on Hall gain. Hall gain varies significantly depending on operating conditions including temperature and bias current, and therefore the current probe measurement system shown in FIG. 1 is subject to overall measurement errors because of the variation in Hall gain.
The value of T.sub.real can be derived implicitly in a calibration sequence, in which a current of known value is passed through the primary conductor 8 and the gain A.sub.s of the scaling amplifier 50 is adjusted so that the current value indicated by the display device 56 is equal to the known value. However, this calibration procedure is subject to the disadvantage that since Hall gain varies significantly depending upon operating conditions, the system must be recalibrated frequently, and particularly whenever the temperature of the Hall device changes significantly. In principle, it would be possible to calculate the value of T.sub.real, but hitherto it has not been practicable to measure the Hall gain each time the operating conditions change.